In an automotive differential gear and a transfer apparatus for a four-wheel drive vehicle, it has heretofore been proposed to use a construction in which tapered roller bearings are provided respectively at a pinion gear-side portion (one axial end portion) of a pinion shaft and an anti-pinion-gear-side portion (the other axial end portion) of the pinion shaft, and the pinion shaft is rotatably supported on an inner peripheral surface of a case by the two tapered roller bearings (see Patent Literatures 1 and 2).
However, in this pinion shaft-supporting construction, a radial load and a thrust load simultaneously act on the pinion gear-side tapered roller bearing, and besides the proportion of the thrust load is high, and therefore this tapered roller bearing has a large contact angle.
Therefore, a large frictional resistance acts on this tapered roller bearing, so that a running torque becomes large, and as a result the efficiency of the differential gear tends to be lowered.
Therefore, there has been proposed a pinion shaft-supporting bearing apparatus in which instead of a tapered roller bearing, a double row rolling bearing is used as a pinion gear-side bearing supporting a pinion shaft (see Patent Literature 3).
A differential gear, in which a pinion gear-side portion (free end portion) of a pinion shaft is supported on an inner peripheral surface of a case by this double row rolling bearing, will be described with reference to FIG. 11.
The differential gear 1 includes the differential case 2. The differential case 2 comprises a front case 3 and a rear case 4, and the two cases 3 and 4 are joined together by bolt/nut arrangements 2a. 
Bearing-mounting annular walls 27A and 27B are formed within the front case 3. The differential case 2 contains a differential speed change mechanism 5 interlocking right and left wheels in a differential manner, and the pinion shaft 7 having a pinion gear 6.
The pinion gear 6 is in mesh with a ring gear 8 of the differential speed change mechanism 5.
A shaft portion 9 of the pinion shaft 7 is stepped to increase in diameter toward the pinion gear 6. A pinion gear-side portion of the shaft portion 9 of the pinion shaft 7 is rotatably supported on the annular wall 27A of the front case 3 through the double row rolling bearing 10.
An anti-pinion-gear-side portion (remote from the pinion gear 6) of the shaft portion 9 of the pinion shaft 7 is rotatably supported on the annular wall 27B of the front case 3 through a double row rolling bearing 25.
The front case 3 has an oil circulating passageway 40 formed between its outer wall and the annular wall 27A, and an oil inlet 41 of the oil circulating passageway 40 is open toward the ring gear 8, and an oil outlet 42 of the oil circulating passageway 40 is open to a region between the annular walls 27A and 27B.
A closure plate 37 is inserted into the front case 3 through a smaller-diameter opening thereof, and is fitted on the shaft portion 9 of the pinion shaft 7.
A barrel portion 44 of a companion flange 43 is spline-fitted on the shaft portion 9, and an end surface of this barrel portion 44 is held against the closure plate 37. An oil seal 46 is mounted on the barrel portion 44 of the companion flange 43.
A seal protection cup 47 is attached to the smaller-diameter opening portion of the front case 3.
A nut 49 is threaded on a threaded portion 48 of the shaft portion 9, thereby imparting a predetermined preload to rows 17 and 18 of rolling elements of the double row rolling bearing 10 and rows 32 and 33 of the double row rolling bearing 25.
In the above construction, the nut 49 is threaded on the threaded portion 48, so that an inner ring 13 of the double row rolling bearing 10 and an inner ring 14 of the double row rolling bearing 25 are held between an end face of the pinion gear 6 and an end surface of the companion flange 43 in the axial direction, thereby imparting the predetermined preload to the rows 17 and 18 of rolling elements of the double row rolling bearing 10 and the rows 32 and 33 of rolling elements of the double row rolling bearing 25 through the closure plate 37 and a plastic spacer 23.
Here, the double row rolling bearing 10 and the double row rolling bearing 25 are provided respectively on at least two axially-spaced portions, that is, the pinion gear-side portion and the anti-pinion-gear-side portion of the pinion shaft 7, and jointly form a rolling bearing apparatus supporting the pinion shaft 7 on the annular walls 27A and 27B each serving as the inner peripheral surface of the case.
The conventional double row rolling bearing 10 will now be described with reference to FIG. 12. This double row rolling bearing 10 is a double row angular contact ball bearing, and comprises an outer ring 11 having a larger-diameter raceway 11a and a smaller-diameter raceway 11b which are arranged in two rows in the axial direction, the inner ring 13 having a larger-diameter raceway 13a and a smaller-diameter raceway 13b which are arranged in two rows in the axial direction and are opposed respectively to the larger-diameter raceway 11a and the smaller-diameter raceway 11b in the radial direction, the larger-diameter row 17 of rolling elements disposed between the larger-diameter raceways 11a and 13a, the smaller-diameter row 18 of rolling elements disposed between the smaller-diameter raceways 11b and 13b, a larger-diameter cage 19 holding the larger-diameter row 17 of rolling elements, and a smaller-diameter cage 20 holding the smaller-diameter row 18 of rolling elements.
In this double row rolling bearing 10, the inclinations of contact angles of the larger-diameter and smaller-diameter rows 17 and 18 of rolling elements for the respective raceways (11a and 13a) and (11b and 13b) are in the same direction.
Among the bearing parts of the double row rolling bearing 10, the inner ring 13, the rows 17 and 18 of rolling elements and the cages 19 and 20 are assembled together to form an inner ring assembly 21, and this inner ring assembly 21 is inserted into the outer ring 11 through a larger-diameter opening thereof to be mounted therein.
Namely, the outer ring 11 is fixed to an inner peripheral surface of the annular wall 27A with its axis disposed horizontally. The inner ring assembly 21 is fitted on the pinion shaft 7. The pinion shaft 7 having the inner ring assembly 21 fitted thereon is inserted horizontally into the larger-diameter opening of the outer ring 11, thereby assembling the outer ring 11 and the inner ring assembly 21 together in the horizontal direction. By doing so, the pinion gear-side portion of the pinion shaft 7 can be supported on the annular wall 27A through the double row rolling bearing 10.
However, the double row rolling bearing 10 of the above construction has the following problems.
These problems will be described with reference to FIGS. 13 and 14. In the drawings, the upward-downward direction is defined as the vertical direction, and the right-left direction is defined as the horizontal direction. The outer ring 11, the pinion shaft 7 and the inner ring 13 of the inner ring assembly 21 have their respective axes 31 coinciding with one another. FIG. 13 shows a condition before the inner ring assembly 21 is mounted in the outer ring 11. The outer ring 11 is mounted on the inner peripheral surface of the annular wall 27A (that is, the inner peripheral surface of the case) with its axis disposed horizontally. On the other hand, the inner ring assembly 21 is fitted on the pinion shaft 7. The distance from the axis 31 of the outer ring 11 to a vertically-lower portion of an inner peripheral edge 11c of the larger-diameter opening of the outer ring 11 is represented by D0.
In the inner ring assembly 21, the rows 17 and 18 of rolling elements are circumferentially disposed on the raceways 13a and 13b of the inner ring 13, respectively, and are held by the cages 19 and 20, respectively.
In this condition, any load, that is, any external force, acts on the rows 17 and 18 of rolling elements. A radial clearance exists between the row 17 of rolling elements and the raceway 13a of the inner ring 13, and also a radial clearance exists between the row 18 of rolling elements and the raceway 13b of the inner ring 13. Therefore, each of the rolling element rows 17 and 18 moves in a direction to reduce the vertically-upper radial clearance because of the deadweight, so that the vertically-lower radial clearance increases.
Namely, the rolling element row 17 as well as the rolling element row 18 drops downward in an amount corresponding to the radial clearance. As a result, the vertical distance D1 from the axis 31 to a vertically-lowermost end of the larger-diameter row 17 of rolling elements often becomes larger as compared with the vertical distance D0 from the axis 31 of the outer ring 31 to the vertically-lower portion of the inner peripheral edge 11c of the larger-diameter opening of the outer ring 11.
In the case where this distance relation is established, the larger-diameter row 17 of rolling elements are caused to abut against the inner peripheral edge portion 11c of the larger-diameter opening of the outer ring 11 when the pinion shaft 7 is inserted into the outer ring 11 through the larger-diameter opening thereof as shown in FIG. 14.
The inner peripheral edge 11c of the larger-diameter opening of the outer ring 11 has a generally right-angled corner-shape, and therefore when the speed of mounting (or inserting) of the pinion shaft 7 into the larger-diameter opening of the outer ring 11 is high, or when this mounting force is high, there is a possibility that the rolling elements forming the rolling element row 17 are easily damaged at their surfaces by the inner peripheral edge 11c of the larger-diameter opening of the outer ring 11.
And besides, when those rolling elements of the rolling element row 17 which have been damaged at their surfaces roll on the raceway 13a of the inner ring 13 at high speed, a large vibration and sound are produced, and also the raceway 13a of the inner ring 13 is subjected to damage such as flaking, etc., which invites a shortened life of the double row rolling bearing 10 at a premature stage.
Patent Literature 1: JP-A-9-105450 Publication
Patent Literature 2: JP-A-10-220468 Publication.
Patent Literature 3: JP-A-2004-190728 Publication